Dynamic Behavior of Cross Tunnels in Sandy Soils of Different Relative Densities

Cross-tunnel behavior under dynamic loadings is essential for practical design. A complex response of vertically stacked cross tunnels in a soil medium is examined in the present article. The analysis involves sandy soil of different relative densities for a soil–tunnel system under dynamic loading conditions. This work examines the impact of tunnel lining thickness, flexibility ratio, cross-tunnel pillar depth, soil relative density, and variation in the amplitude of input motion on the dynamic behavior of cross tunnels. Harmonic waves are used to analyze five different parameters that define the behavior of cross tunnels in sandy soil. The results of the analyzed responses indicate that relatively rigid cross tunnels are preferable in most cases, particularly when the second tunnel is located below the first one. The results show a drastic variation of 45% in the surface amplification ratio between single and cross tunnels in loose sand and a maximum horizontal convergence of 1.7% of tunnel radius for relatively flexible cross tunnels in dense sand. Cross tunnels at shallow depths experience a 62.9% increase in settlement compared with single tunnels in dense sand. Moreover, Type 1 relatively flexible cross tunnels experience a 15% higher dynamic bending moment than the single tunnel case. A drastic jump of 25.7% is observed in axial thrust values for relatively rigid cross tunnels, even though the thrust magnitude is lower. For residual earth pressure, Type 2 relatively flexible cross tunnels are preferable for dynamic events. It is also found that a direct correlation between dynamic surface displacement or any other soil–tunnel response parameter and the flexibility ratio of the tunnel can be misleading, unless the changing variables of flexibility ratio formulation are known. This investigation shows some of the critical combinations of cross tunnels required for the dynamic design of tunnel networks in regions with sandy soil.